1. Field of the Invention
The present invention relates to a system and method for providing accurate data for use in performing Time of Flight (TOF) measurements in communications between nodes in a wireless network, where nodes employ orthogonal frequency division multiplexing (OFDM) modems. Specifically, the system and method provides an algorithm for use with the linear time shift property of the data demodulation in OFDM implementations to correct the time shift and allow accurate calculation of synchronization points for TOF calculations.
2. Description of the Related Art
Wireless communication networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks are commonly referred to as “cellular networks”, because the network infrastructure is arranged to divide the service area into a plurality of regions called “cells”. A terrestrial cellular network includes a plurality of interconnected base stations, or base nodes, that are distributed geographically at designated locations throughout the service area. Each base node includes one or more transceivers that are capable of transmitting and receiving electromagnetic signals, such as radio frequency (RF) communications signals, to and from mobile user nodes, such as wireless telephones, located within the coverage area. The communication signals include, for example, voice data that has been modulated according to a desired modulation technique and transmitted as data packets.
In recent years, a type of mobile communications network known as an “ad-hoc” network has been developed for use by the military. In this type of network, each mobile node is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other mobile nodes, such as those on the public switched telephone network (PSTN), and on other networks such as the Internet. Details of these advanced types of ad-hoc networks are described in U.S. patent application Ser. No. 09/897,790 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, filed on Jun. 29, 2001, in U.S. patent application Ser. No. 09/815,157 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, filed on Mar. 22, 2001, and in U.S. patent application Ser. No. 09/815,164 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, filed on Mar. 22, 2001, the entire content of each being incorporated herein by reference.
As can be appreciated by one skilled in the art, network nodes transmit and receive data packet communications in a multiplexed format, such as time-division multiple access (TDMA) format, code-division multiple access (CDMA) format, or frequency-division multiple access (FDMA) format, which enables a single transceiver at the base node to communicate simultaneously with several mobile nodes in its coverage area. Yet another multicarrier transmission technique includes orthogonal frequency division multiplexing (OFDM). This technique divides the available spectrum into many carriers, each one being modulated by a low rate data stream. OFDM is similar to FDMA in that the multiple user access is achieved by subdividing the available bandwidth into multiple channels that are then allocated to users. However, OFDM uses the spectrum much more efficiently by spacing the channels much closer together. This is achieved by making all the carriers orthogonal to one another, preventing interference between the closely spaced carriers.
In such multicarrier transmissions, there unfortunately exists a degree of multiuser interference. As discussed in an article by Victor Shtrom entitled “CDMA vs OFDM In Broadband Wireless Access”, the entire content of which is being incorporated herein by reference, the use of synchronous CDMA can lower multiuser interference. Synchronization lowers interference by time-aligning the chip boundaries of all users, however, multipath destroys synchronicity due to reflections from buildings and other topographic obstacles. Signals travel different distances while in route to the receiver and thus arrive out of sync. As noted by Shtrom, it is also difficult to synchronize users because the differing time-of-flight (TOF) delays depend on user location.
Additionally, in OFDM implementations, the time frame synchronization requirements for data demodulation also may not be suitable for TOF measurements. For example, Time of Flight measurements in certain applications require accurate measurement of the round-trip delay from source to destination, and back to the source. The destination must receive the signal and transmit a signal back to the source with a high degree of timing accuracy. In other applications not requiring round trip delay measurements, timing accuracy for signal transmission and receipt is still crucial. However, in an OFDM modem transmission, the subcarrier modulation is performed with a Fast Fourier Transform (FFT) that typically includes a time shift between Inverse Fast Fourier Transform (IFFT) and guard interval insertion at transmission, affecting TOF measurements. This time shift could thus adversely affect timing accuracy.
Accordingly, a need exists for a system and method to provide accurate frame synchronization for communications between nodes in a wireless network, where nodes employ orthogonal frequency division multiplexing (OFDM) modems.